In this technological world it is often considered desirable to be able to detect and respond to elements passing a predetermined location. Numerous examples exist in traffic control wherein it may be desired to identify the number of vehicles passing a given intersection and/or to determine the number of vehicles waiting to enter a given intersection. Devices may also be used to count people as they enter and/or leave a given location; to count manufactured articles coming down an assembly line; to identify unauthorized motion in a protected area; and a wide variety of other applications which are familiar to those acquainted with the applicable arts.
A wide variety of techniques have been employed in the design and operation of such devices. For example, light beams and photodetector cells have been employed; weight detectors are frequently used; light reflecting devices are used, and in applicable situations, the presence of an article may be detected due to its effect on a magnetic field.
The structure to be described herein is of the class whose presence is detected in response to the effect that the object has on a magnetic field. Although the techniques and structures disclosed and described herein obviously have more general utility, they are specifically described in a railroad setting wherein it is desired to be able to detect and respond to the presence and/or passage of a railroad wheel passing a predetermined location. Detecting changes in magnetic field strength in response to the presence or absence of a train wheel is not broadly new, and a substantial number of devices and techniques have been developed for this purpose. Examples of prior devices employing a magnetic circuit may be seen in the Smith U.S. Pat. No. 3,562,603 or the Bolton U.S. Pat. No. 3,697,745 issued Feb. 9, 1971 and Oct. 10, 1972, respectively, and assigned to the same assignee as the present application. A related device for responding to detection signals is shown in the Auer U.S. Pat. No. 3,601,664 issued Aug. 24, 1971 and assigned to the same assignee as the present application.
These devices may be used in the railroad industry for a wide variety of purposes. For example, with associated equipment, they may be used to detect a train entering a station and provide a signal of where to stop so that the cars are in the most propitious location. They may also be used in a switch yard to count cars and assist in the makeup of a train and/or to prevent placing too many cars on a specific track. Detectors are sometimes used in pairs, spaced apart a known distance so that the time differential in triggering the two detectors may be used to calculate train speed and/or direction of motion. Detectors may also be used to identify a train approaching a grade crossing in order to provide suitable warning signals. Other applications in the railroad industry will readily occur to those with appropriate training and experience; and those with other training and experience will be able to identify utility in their art.